1. Field of the Invention
The present invention relates generally to gas spring-assisted counterbalance systems, and more particularly to a counterbalance system of that type in which a supplemental mechanical spring is connected in series with the gas spring to isolate the gas spring unit from recurrent short cycles due to system vibrations or the like.
2. The Prior Art
Gas springs of the generic type disclosed in U.S. Pat. No. 3,207,498 have found wide acceptance in many industries as convenient and efficient devices for pneumatically counterbalancing loads such as the weight of doors, windows and other closure members during opening and/or closing movements. This is especially true in the automobile industry, where gas springs have been widely used as counterbalance devices for trunk decks, hatchback doors and windows, engine compartment covers, and so forth.
Although gas springs function well in such applications, they remain functional only so long as the integrity of the high pressure gas seal and bearing assembly is maintained. Such seal and bearing assemblies have a finite lifetime, however, and are susceptible to premature failure if subjected to undue cycling (longitudinal movement of the piston rod relative to the seal and bearing assembly) of the gas spring. This situation has been found to exist in certain automotive installations, for example, where road-induced vibrations result in undesirable numerous short cycles of the gas spring. In another example, manufacturers of hatchback models sometimes provide for the hatchback to open slightly to relieve air pressure within the car when a passenger door is closed, thus permitting easy closing of the door. As a consequence, the gas spring or springs attached to the hatchback are cycled every time a passenger door is used, with the result that the rated cycle lifetime of the gas springs will be exceeded much sooner than would otherwise occur.
While it is known to isolate fluid dampers and gas springs with elastomeric bushings at the attachment points, this technique is not satisfactory for the aforementioned applications. Such bushings add an additional spring effect in series with the gas spring, which additional effect is always present and can lead to undesirable bounce in the overall spring system in the extended position. Further, the displacement of the elastic bushings is too small to accommodate other system variables such as manufacturing and assembly tolerances, overtravel requirements, etc., which affect the distance between the gas spring attachment points in the retracted position of the gas spring.
As will be appreciated, therefore, a need exists in the art for a gas spring-assisted counterbalance system which isolates the gas spring component from undesirable, recurrent short cycles that impair the useful lifetime of the spring. The present invention fulfills this and other requirements of the prior art.